Azines diazonium salts

Mesoionic systems may be readily substituted by electrophiles. Thus the thiazolo mesoion (342) will couple with diazonium salts despite their relatively weak electrophilicity (80KGS621). Substitution in a fused heteroaromatic betaine azine ring, e.g. (343), also takes place with ease. The resonance form (344) of the mesoion (343) shows that the electrophile will attack at C-6. The substitution in this position is also predicted by MO calculations (73JHC487). Similarly the pyridine ring in pyridinium olates is active towards electrophiles and is substituted in the positions ortho and para to the olate function. Bromination of the 5-methyl derivative (321 R = Me) occurs exclusively in the 7-position which is rationalized via the intermediate (345). In the absence of a 5-substituent, attack in either the 5- or 7-position occurs the dibromide is readily formed. No bromination in the thiazole ring is observed. The 2-bromo derivative (346) has been made, however, by condensation between the appropriate mercaptopyridine and 1,1,2,2-tetrabromoethane. 

Azine approach. Diazotization of 2-amino-3-hydroxypyridines might be expected to yield the bicyclic ring system (631) by bond formation between the oxygen and the diazonium group in the first formed diazonium salt fused 1,2,3-thiadiazoles and -triazoles are formed by this type of reaction. Compounds prepared by this method, however, are highly unstable and have spectroscopic properties consistent with the pyridodiazooxide structure (630) (74CS(6)222>. 

Like pyridine, the azines are not normally attacked by diazonium cations, but since coupling reactions seldom involve extensive protonation of the substrate, the reactions become viable provided that sufficient numbers of activating groups are present, and if reactive diazonium salts are used. Pyridazin-3,4(l//,2//)-diones are among the few pyridazines... 

A method using polyvinylpyridinium hydrofluoride (PVPHF), as fluoride source, at low temperature was suitable for the relatively sensitive 2-deoxy-nucleosides. This low temperature reaction was probably successful due to the instability, i.e. high reactivity, of diazonium salts at the a-position of azines, and may not be general for other heterocycles. 

In this Section only those hydrazine syntheses are discussed that occur with formation of an N-N bond (TV-amination). For syntheses of substituted hydrazines by reduction of hydrazones, azines, iV-nitroso compounds, and diazonium salts the Sections relevant thereto should be consulted. 

Hammett equation, applied to azines, 217 Hetarynes, 121-143 contrasted with arynes, 125 Heteroaromatic compounds, covalent hydration of, 1-41 "pKa generalizations for, 48-61 Heterocyclic acids, pH-rate profile for, 67 Heterocyclic diazonium compounds, 241 Heptaazanaphthalenes, 393 Hexaazanaphthalenes, 393 Hippuroflavin, 80 Hydrated salts, isolation of, 16 Hydrates, alcoholates from, 16 isolation of, 16... 

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